@mitu I made another test replacing the SSD ( Kingston SSD KC3000, 2048 GB, M.2 2280, NVMe PCIe Gen 4.0, Read: 7000MB/s, Write: 7000MB/s, SKC3000D/2048G) for a SSD "Adata Legend 740 NVMe, 250GB, PCI Express 3.0, M.2"
ALEG-740-250GCS-6.jpg
And this SSD disk works!! (now I can see several folders inside "retropie-mount" folder, "BIOS", "roms", etc)... but is a very tiny SSD compared with the Kingston one, maybe it worked because this is a PCIE 3.0 and the Kingston is PCIE4.0? maybe raspberry pi 4 doesn't support PCIe 4.0 because that technology needs more power? I used the same enclosure for both SSDs (ORICO M.2 NVMe SSD Enclosure to USB-C PCIe Adapter 10Gbps USB 3.2 Gen2 for NVMe M-Key/M+B Key SSD 2230/2242/2260/2280)